A day after officially taking over the department early Wednesday at a private midnight ceremony, Mr. Bratton stood before a packed hall at Police Headquarters in Lower Manhattan and promised a new era of policing in New York City.

He began his speech by saying, "Who says you can't come home again."

During his address, Mr. Bratton promised "a collaboration unlike any that we have ever seen in this city" between the police and the people they serve.

He said he would seek to understand the "disconnect" that had prevented the Police Department from being celebrated for its accomplishments in delivering a safe city in recent years and instead was met with distrust in many communities. "That's why I came back," he said.

A law enforcement leader with international renown, Mr. Bratton returns to a department he last ran in the 1990s, when the task before him was to reduce crime. On Wednesday he succeeded Raymond W. Kelly, whom he also replaced in 1994.

The city has grown safer since then, recording fewer than 340 murders in 2013 compared with about 1,500 in 1994, when he was last sworn in. But the practices of the Police Department, especially in minority neighborhoods where crime stubbornly remains, became the focal point of Mr. de Blasio's successful campaign. Promises to reform the department was instrumental to his victory.

Mr. Bratton, who becomes the city's 42nd police commissioner, must now try to make good on the pre-election rhetoric of the man who picked him to better relations between the Police Department and minorities, especially to change the stop-and-frisk practices that have sown distrust in many quarters. Mr. Kelly, who has staunchly defended the practices as being a legitimate anti-crime tactic, did not attend Thursday's ceremony.

Mr. Bratton will most likely draw on his experience in Los Angeles, where he led a police force, which had become synonymous with corruption and brutality, out from under a federal consent decree in the 2000s. He reached out there to the department's critics early on, and won over most of them by the time he stepped down in 2009, despite a doubling in the number of stop-and-frisk encounters during his tenure.

Mr. Bratton has applied a similar playbook so far in New York, meeting with minority leaders and the department's staunchest critics almost immediately after his selection was announced last month.

Addressing the rank-and-file officers on Thursday, Mr. Bratton sought to reassure them that he would be a strong defender of their interests, as well. How much the officers on the street support him will probably be a significant factor in maintaining a level of safety that New Yorkers have come to expect.

After leaving the New York Police Department in 1996, Mr. Bratton took a number of security positions and founded his own consulting company; he was widely sought as an adviser for police departments across the country and the world. In recent years, he served as chairman of Kroll, the private security and investigations company, and on the boards of Motorola Corporation and ShotSpotter, a gunshot-detection technology company. In those positions, he has emphasized the importance of collaboration in business and law enforcement success.

Mr. Bratton closed by addressing 10 members of the most recent Police Department academy class, who graduated last week and were seated at the front of the hall, which was filled with policing leaders from around the country.

"Everyday you get the opportunity to do good," he said. "It doesn't get any better than that."